1. Field of the Invention
The present invention is related to a device and method for a multiuser detection in a DS-CDMA(Direct Sequence-ode Division Multiple Access) system, and more particularly, to a device and method for a decorrelating multiuser detection in a DS-CDMA system which allows real time removal of an MAI(Multiple Access Interference) occurred in the DS-CDMA system.
2. Discussion of the Related Art
The DS-CDMA system is a system in which a transmitter modulates and transmits its message by making a direct band spreading with a pseudo noise (PN) code or a spreading signal(code) assigned to multiusers individually for distinguishing between the multiusers, and a receiver restores an originally transmitted message by receiving the transmitted signal and by despreading the transmitted signal with the spreading code. Since the DS-CDMA system has many merits, such as strong in multipath fading, good utilization of voice activity cycles, availability of soft handoff between base stations, strong in jamming, and reuse of one frequency band, which reuse allows the DS-CDMA system to have a greater capacity over background art systems, the DS-CDMA systems has been spot lighted in implementation of cellular and personal communications, recently. Despite the aforementioned merits of the DS-CDMA system, because its performance is restricted by MAI occurred at multiuser reception or great SNR(signal to noise ratio), ceaseless efforts have been concentrated on removing the MAI, and thus various multiuser detectors have been proposed up to now.
As a typical one of the multiuser detector, there is a decorrelating multiuser detector as shown in FIG. 1, provided with a matched filter block 10 having a plurality of multipliers (CO.sub.1 -CO.sub.K), a plurality of integrators (I.sub.1 -I.sub.K), and a plurality of switches (SW.sub.1 -SW.sub.K) for despreading a received signal r(t) with multiuser' spreading codes to provide a sample of each message of the multiuser, a R.sup.-1 filter 11 for filtering the samples in the matched filter block 10 to remove an MAI signal included in each of the samples of the multiuser's messages, and a binary data determining part 12 having a plurality of determiners(DC.sub.1 -DC.sub.K) for comparing plural outputs (Z.sub.1 -Z.sub.K) corresponding to the multiuser of the R.sup.-1 filter 11 with a threshold voltage to determine binary outputs (b.sub.1 -b.sub.K).
The operation of the background art decorrelating multiuser detector will be described
The received signal r(t) input to the decorrelating multiuser detector is despreaded through the plurality of matched filters in the matched filter block 10 having the plurality of multipliers (CO.sub.l CO.sub.K), integrators (I.sub.1 -I.sub.K) and switches (SW.sub.1 -SW.sub.K) and each of the original messages of the multiuser is recovered. The outputs (y.sub.1 -y.sub.K) of the matched filter block 10 can be expressed as a matrix as follows. EQU y=RAb+n,
##EQU1##
Where,
Y=(y.sub.1, (y.sub.2, . . . , y.sub.K).sup.T ##EQU2## PA1 b=(b.sub.1, b.sub.2, . . . , b.sub.K).sup.T PA1 n=(n.sub.1, n.sub.2, . . . , n.sub.K).sup.T
Where, a.sub.i is a received amplitude of an with multiuser, A is an amplitude matrix of the received signal, b is a bit vector of a transmitted data, n is a Gaussian noise vector, and an element P.sub.ij, in the R matrix represents a cross-correlation coefficient between with and jth user spreading codes.
If an output y from the matched filter block 10 as expressed in equation (1), is provided to the R.sup.-1 filter 11, an output Z expressed as the following equation (2) can be obtained. EQU Z=R.sup.-1 y=Ab+R.sup.-1 (2) EQU Z=(Z.sub.1,Z.sub.2, . . . Z.sub.K,)
The output Z of the R.sup.-1 filter 11, expressed as equation (2), sends to the binary data determining part 12 having the binary determiners DC.sub.1 -DC.sub.K where the binary data of multiuser is recovered.
The background art decorrelating multiuser detector can completely remove the MAI signal caused by the cross-correlation value (not 0) between multiuser's spreading codes and included in the output y of the matched filter 10 by using the R.sup.-1 filter and thus can improve quality of the received signal. However, the R.sup.-1 filter is required to compute an inverted matrix of R as shown in the following equation (3). This computation becomes the more complex as the dimension of the matrix becomes the greater as a number of the users increases. ##EQU3##
In this equation (3), the diagonal element b.sub.11 is expressed as b.sub.11 =1-(K-1)(K-2)/2 second order term of the cross-correlation coefficient +O(P.sup.3), the diagonal element b.sub.22 is expressed as b.sub.11 =1-(K-1)(K-2)/2 second order term of the cross-correlation coefficients +O(P.sup.3), and the other diagonal elements b.sub.ij are expressed in the same manner. The non-diagonal element b.sub.12 is expressed as b.sub.12 =-P.sub.12 +(K-2) second order term of the cross-correlation coefficients +O(P.sup.3) and the non-diagonal element b.sub.13 is expressed as b.sub.13 =-P.sub.13 +(K-2) second order term of the cross-correlation coefficients +O(P.sup.3). The other non-diagonal element b.sub.ij is expressed in the same manner. The O(P.sup.3) denotes a polynomial of the cross-correlation coefficients having a third order term and greater. Therefore, the R.sup.-1 filter has a problem in that a circuit can not be realized actually due to the excessive amount of computation.
In order to solve the aforementioned problem, Moshavi et al. disclosed a paper titled "Multistage Linear Receiver for DS-CDMA Systems" (International journal of wireless Information Network, vol. 3, No. 1, 1996). Referring to FIG. 2, the decorrelating multiuser detector disclosed by the paper is provided with a match filter block G for multiplying a received signal r(t) transmitted from multiuser with a conjugate complex g.sub.0 *(t).about.g.sub.K-1 *(t) of the spreading code and passing through a low pass filter(LPF) for despreading the received signal r(t), a least mean square error detecting part having a multistage R, R,--of cross-correlation coefficient matrix implementing blocks R for multiplying, and summing the spreading code g.sub.0 (t).about.g.sub.K-1 (t) from transmitters to outputs y.sub.0.about.y.sub.K-1 of the matched filter block G, multiplying to the conjugate complex g.sub.0 *(t).about.g.sub.K-1 *(t) of the spreading code from the transmitters, and passing through low pass filters(LPF), wherein the outputs y.sub.0.about.y.sub.K-1 of the matched filter block G are multiplied to weighted values W.sub.0, W.sub.1, W.sub.2,--which are coefficients calculated on least mean square error basis of outputs of each stage, compensated for a time delay at each stage through a delay Tb which delays for one bit, and added of a signal from the multistage of the cross-correlation coefficient matrix implementing blocks R, R,--in the least mean square error detecting part, to obtain an approximate output d of the background art decorrelating detector The aforementioned example shows the least mean square error detecting part having two stages of the cross-correlation coefficient matrix implementing blocks for simplicity. According to the aforementioned system, because values of Ry, R.sup.2 y,--are generated every time one stage of the cross-correlation coefficient matrix implementing block R in the least mean square error detecting part is passed, which are then multiplied of the weighted values W.sub.0, W.sub.1, W.sub.2,--and time compensated, to obtain d, an approximate inverted matrix R of the cross-correlation coefficient matrix R can be implemented.
However, because the background art should calculates the weighted values W.sub.0, W.sub.1, W.sub.2,--on a least mean square error basis again under an ambient in which the cross-correlation coefficient matrix changes quickly according to time, such as a system which uses a long code, or of an asynchronous type, or a number of the multiuser changes quickly, the background art has problems as that not only an additional block is required for calculating the above, but also a real time implementation of the same in actual system is difficult.